"coin, money in the form of coins, metallic money as a medium of exchange" (as opposed to paper money or bullion), 1670s, a noun use from the Medieval Latin phrase in specie "in minted coins" (attested in English by 1610s). This was a specialized sense in reference to money; the broader and classical meaning of in specie was "in kind; in the real, precise, or actual form" (1550s in English). Latin specie is ablative singular of species "kind, sort; appearance, form" (see species).
c. 1500, "consisting of money;" 1620s, "relating to money," from Latin pecuniarius "pertaining to money," from pecunia "money, property, wealth," from pecu "cattle, flock," from PIE root *peku- "wealth, movable property, livestock" (source of Sanskrit pasu- "cattle," Gothic faihu "money, fortune," Old English feoh "cattle, money").
Livestock was the measure of wealth in the ancient world, and Rome was essentially a farmer's community. That pecunia was literally "wealth in cattle" was still apparent to Cicero. For a possible parallel sense development in Old English, see fee, and compare, evolving in the other direction, cattle. Compare also Welsh tlws "jewel," cognate with Irish tlus "cattle," connected via the notion of "valuable thing," and, perhaps emolument.
An earlier adjective in English was pecunier (early 15c.; mid-14c. in Anglo-French), from Old French; also pecunial (late 14c.).
German tasche "pocket" is from the same Vulgar Latin source (via Old High German tasca), with presumable sense evolution from "amount of work imposed by some authority," to "payment for that work," to "wages," to "pocket into which money is put," to "any pocket."